Abstract

Coalbed methane is a natural by-product of the coalification (thermal maturation and its progenitors) of humic materials that contain type III vitrinitic kerogen. Although a large percentage of the methane generated by the coalification process escapes to the surface or migrates into adjacent reservoir rocks, a portion is trapped within the coal itself, primarily adsorbed on or absorbed within micropores of the coal.

Although coalbed methane exists in all coal regions of Wyoming, some of the State's coal deposits are shallow and too thermally immature to have generated substantial amounts of thermogenic gas. However, many of these shallow coals do have biogenic (microbially-generated) methane entrapped in them. The most significant quantity of biogenic methane exists in the relatively shallow, thick coal beds in the Powder River Coal Field. While methane content per ton of coal is relatively low in this basin, the individual coal bed thickness as well as the aggregate thickness of the shallow Tertiary coal measures account for the accumulation of large quantities of biogenic coalbed methane. In contrast, Cretaceous and some Tertiary coal beds buried in the structurally deep portions of many Wyoming basins are much more thermally mature and no doubt have generated and do contain large volumes of thermogenic methane.

The coalbed methane play in the Powder River Coal Field is currently the most active gas play in the United States. In September of 1999, monthly production had reached 5.1 billion cubic feet from 1,169 producing wells, with 807 shut-in wells awaiting pipeline connections. Production should reach 60 billion cubic feet in 1999.

Renewed interest in coalbed methane has expanded to most of the other Wyoming coal fields outside the Powder River Coal Field. This interest is due, in part, to spillover from success in the Powder River Basin and, in part, due to increased exploration for more conventional (thermogenic) coalbed methane targets.

Estimated recoverable gas resources from coal beds for all Wyoming basins total 16.74 trillion cubic feet. This total alone is over 3 trillion cubic feet more than Wyoming's proved reserves of 13.65 trillion cubic feet of dry natural gas in traditional natural gas fields. The total is also about 70 percent of the cumulative total natural gas production from Wyoming through 1998. From these comparisons, it is obvious that coalbed methane will be an important resource in Wyoming for many years.